1. Field of the Invention
The present invention relates generally to semiconductor devices, methods of manufacturing the same, and electronic devices, and more particularly to a semiconductor device including a resistive element, a method of manufacturing the same, and an electronic device employing the same.
2. Description of the Related Art
Conventionally, resistors such as those using a Si impurity diffusion layer and those formed of polysilicon have been widely used for forming a resistive element incorporated into semiconductor devices and electronic devices. The resistance of such a resistive element is controlled by, for instance, previously providing multiple interconnection lines that short-circuit the resistors forming the resistive element, and cutting off one or more of the interconnection lines in accordance with the results of circuit measurement. In this case, in part of the resistive element where the interconnection lines are cut off, the resistors function so as to increase the resistance of the resistive element.
Such a method of controlling the resistance of the resistive element entails the following problems.
The first problem is that the accuracy of the resistance control is low so that the obtained resistance varies greatly. The second problem is that it is difficult to accurately control the rate of change in the resistance of the resistive element with respect to a change in temperature, that is, it is difficult to control the temperature coefficient of the resistive element with accuracy. The third problem is that the control of the resistance can be performed only in a direction to increase the resistance and it is difficult to correct the control once it has been performed because the resistance is controlled by cutting off interconnection lines.
The following methods have been proposed to solve the above-described problems.
For instance, in order to solve the problem that it is difficult to control the temperature coefficient of the resistive element, Japanese Laid-Open Patent Application No. 5-75445 discloses a device and method for correcting a variation in the oscillation frequency of a CPU in the case of the occurrence of a change in a condition such as temperature. FIG. 1 is a block diagram illustrating the conventional correction device.
Referring to FIG. 1, in the correction device, the positive electrode of a power supply 2 is connected to the terminal Vcc of a CPU 1, and the negative electrode of the power supply 2 is connected to the ground terminal (GND) of the CPU 1. Further, the positive electrode of the power supply 2 is connected via a switch 3 to a line 11, to which a voltage detector 4, a temperature detector 5, and an E2PROM 6 are connected.
Further, the negative electrode of the power supply 2 is connected to a line 12, to which the negative side of each of the voltage detector 4, the temperature detector 5, and the E2PROM 6 is connected.
A series circuit of a resistor 7 and a capacitor 9 and a series circuit of a resistor 8 and a capacitor 10 are connected to the lines 11 and 12, respectively, thereby forming the oscillation element of a CR oscillator.
According to the correction device illustrated in FIG. 1, the oscillation frequency of the CR oscillation element at the time of supply voltage and ambient temperature satisfying reference measurement conditions is stored in the E2PROM 6 as a reference oscillation frequency. The CPU 1 corrects the oscillation frequency based on the reference value in accordance with changes in temperature and voltage from the reference measurement conditions, the changes being measured by the temperature detector 5 and the voltage detector 4.
In this case, for instance, with respect to temporal control means realized by performing counting on a system clock signal, the CPU 1 corrects a count in accordance with changes in temperature and voltage.
In addition to the above-described correction device, for instance, Japanese Laid-Open Patent Application No. 2000-91890 discloses a CR oscillation circuit in which a frequency variation due to a change in temperature is controlled. FIG. 2 is a circuit diagram illustrating the conventional CR oscillation circuit.
Referring to FIG. 2, the CR oscillation circuit includes a comparator 21, a reference signal generator circuit 22, a capacitor 23, a resistor 24, and inverters 25 and 26. The capacitor 23 and the resistor 24 are connected in series between the output of the inverter 26 and ground so as to form a charging and discharging circuit.
The inverting input terminal of the comparator 21 is connected to a connection node N1 of the capacitor 23 and the resistor 24. A reference signal, which is the output of the reference signal generator circuit 22, is input to the non-inverting input terminal of the comparator 21.
A reference voltage generated by dividing voltage among resistors 27 through 29 connected in series between a power supply and ground in the reference voltage generator circuit 22 is applied to the comparator 21 via an FET 30 or 31.
In general, if the resistance of the resistor 24 changes because of temperature, there is concern that an output frequency may be subject to change. In the case of the illustrated oscillation circuit, a temperature coefficient employed for the resistor 28 is different in value from those employed for the resistor 27 or 29. Accordingly, it is possible to prevent such a change in the frequency.
That is, as a result of designing the reference voltage generator circuit 22 so that an upper limit voltage VH and a lower limit voltage VL in the case of the CR circuit performing charging and discharging are caused to change by temperature, the effect of a change in the resistance of the resistor 24 due to temperature exerted on the frequency is relaxed, so that a change in the output frequency due to a change in temperature is prevented.
Besides the above-described oscillation circuit, for instance, Japanese Laid-Open Patent Application No. 2002-246849 discloses an amplifier circuit that enables amplification degree to be changed in accordance with a change in temperature. FIG. 3 is a circuit diagram illustrating the conventional amplifier circuit.
Referring to FIG. 3, the amplifier circuit is employed with an input signal Vin being supplied between input terminals 38 and 39. The input terminal 38 is connected to the inverting input terminal of an operational amplifier 35 via resistors 42a and 42b connected in series. The input terminal 39 is connected to the non-inverting input terminal of the amplifier 35 via resistors 43a and 43b connected in series. The output terminal of the amplifier 35 is connected to an output terminal 37.
The inverting input terminal of the amplifier 35 is connected to the output terminal 37 via a resistor 40. The non-inverting input terminal of the amplifier 35 is connected via a resistor 41 to a terminal 36 to which a reference voltage Vref is applied.
In the amplifier circuit, the resistors 42a and 42b are formed to have different temperature coefficients. Accordingly, it is possible to set the temperature coefficient of amplification degree variably by changing the resistance ratio of the resistor 42a to the resistor 42b. 
However, in the case of using the correction device illustrated in FIG. 1, there is a problem in that the configuration of the correction device is complicated so as to increase a circuit in scale.
In the oscillation circuit illustrated in FIG. 2 and the amplifier circuit illustrated in FIG. 3, it is proposed to use multiple resistors of different temperature coefficients in combination. In this case, however, it is difficult in particular to employ a resistor having a positive temperature coefficient for the following reasons, which causes a problem in that it is difficult to achieve a desired temperature coefficient.
For instance, in the case of using the conventionally used resistor using a Si impurity diffusion layer or formed of polysilicon, it is difficult to increase sheet resistance. Therefore, an attempt to obtain a desired resistance in a circuit to be formed increases the resistor size. This makes it difficult to use such a resistor in a normal-size circuit, and also causes a problem in that it is difficult to miniaturize the circuit.
Further, in the case of employing a resistive element using a resistor having a positive temperature coefficient and the conventionally used resistor having a negative temperature coefficient in combination, an attempt to control the temperature coefficient of the resistive element by combining the resistors may result in a problem because the resistors differ greatly in sheet resistance.
For instance, since the resistor having the positive temperature coefficient and the resistor having the negative temperature coefficient differ greatly in size in the circuit, there is a difference in processing accuracy between the resistors in the process of forming the resistors, such as an etching process, and the processing process is also complicated. Further, the resistor having the positive temperature coefficient and the resistor having the negative temperature coefficient differ greatly in the smallest unit of resistor size with which a desired resistance can be obtained, that is, in resistor resolution in the case of controlling the resistance and the temperature coefficient of each resistor. In some cases, this makes it difficult to control the resistance and the temperature coefficient of each resistor with accuracy.
Further, for instance, with a resistor using an N-type well layer, it is possible to form a resistor having a positive temperature coefficient and a high sheet resistance, but it is difficult to form a resistor having a small line width. As a result, the area occupied by the resistor increases, which, in some cases, makes it difficult to miniaturize a circuit using the resistor.
In addition to the above-described problems, there is concern over the following problems.
In the case of employing a resistive element using multiple resistors having different temperature coefficients in combination in the oscillation circuit illustrated in FIG. 2 and the amplifier circuit illustrated in FIG. 3, it is necessary to measure the characteristic related to a temperature coefficient and the resistance of each of the resistors of different types with accuracy. However, there is a problem in that a method and technique for such accurate measurement have not been established completely.
For instance, if a test terminal is provided directly to a resistor in the oscillation circuit in order to measure the temperature characteristic of the resistor, the circuit cannot be expected to operate with accuracy because of the parasitic capacitance of the terminal and noise. Accordingly, monitoring means for measuring the temperature characteristic of the resistor is required separately. However, it is difficult to equalize measurement conditions such as the sheet number and the bias voltage of the monitoring means with those in its operating state in which the monitoring means is incorporated into the actual circuit. Accordingly, the results of the control of the resistance and the temperature coefficient of the resistor are likely to include offsets. For instance, in the oscillation circuit illustrated in FIG. 2, the bias voltage applied to capacitance and resistance before the control is applied to the resistors is different from that after the control is applied to the resistors, so that there is also concern over the effect of the difference.
Further, there is another possible problem in the conventional resistor control method. For instance, it may be difficult to control a resistor in the oscillation circuit illustrated in FIG. 2 or the amplifier circuit illustrated in FIG. 3 only by previously providing multiple interconnection lines that short-circuit the resistors forming a resistive element, and cutting off one or more of the interconnection lines in accordance with the results of circuit measurement.
In this case, one or more of the interconnection lines that short-circuit the resistors are cut off in accordance with measurements obtained by initially measuring the resistance and the temperature coefficient of the resistive element. That is, control is performed only in the direction to increase the resistance in accordance with the measurements. Accordingly, it is difficult to determine the degree of change by changing the resistance in both increasing and decreasing directions at the stage of controlling the resistive element.
Therefore, the resistance of the resistive element is set to a low value in its initial state, and control is performed in a direction to increase the resistance. In the case of, for instance, the oscillation circuit illustrated in FIG. 2, this causes a problem in that if a frequency before the control is offset from a target frequency, the operational delays of the comparator 21 and the inverters 25 and 26 have effects different from those on the controlled frequency, so that an accurate frequency correction cannot be obtained.
Further, there is also a possibility in the amplifier circuit illustrated in FIG. 3 that if the resistance differs greatly between before and after controlling, a change is caused in the characteristic because of the effect of noise, so that a desired characteristic cannot be obtained.
That is, in these circuits, it is difficult to accurately measure the resistance and the temperature coefficient of a resistor in a state close to the actual form of usage, thus causing a problem in that it is difficult to obtain a desired characteristic by controlling the resistance and the temperature coefficient of the resistor with accuracy.